Showing papers by "Lee Hartmann published in 2005"

Disks in Transition in the Taurus Population: Spitzer IRS Spectra of GM Aurigae and DM Tauri

Abstract: We present Spitzer Infrared Spectrograph (IRS) observations of two objects of the Taurus population that show unambiguous signs of clearing in their inner disks. In one of the objects, DM Tau, the outer disk is truncated at 3 AU; this object is akin to another recently reported in Taurus, CoKu Tau/4, in that the inner disk region is free of small dust. Unlike CoKu Tau/4, however, this star is still accreting, so optically thin gas should still remain in the inner disk region. The other object, GM Aur, also accreting, has ~0.02 lunar masses of small dust in the inner disk region within ~5 AU, consistent with previous reports. However, the IRS spectrum clearly shows that the optically thick outer disk has an inner truncation at a much larger radius than previously suggested, ~24 AU. These observations provide strong evidence for the presence of gaps in protoplanetary disks.

Irac observations of taurus pre-main-sequence stars

Abstract: We presented infrared photometry obtained with the IRAC camera on the Spitzer Space Telescope of a sample of 82 pre-main-sequence stars and brown dwarfs in the Taurus starforming region. We find a clear separation in some IRAC color-color diagrams between objects with and without disks. A few "transition" objects are noted, which correspond to systems in which the inner disk has been evacuated of small dust. Separating pure disk systems from objects with remnant protostellar envelopes is more difficult at IRAC wavelengths, especially for objects with infall at low rates and large angular momenta. Our results generally confirm the IRAC color classification scheme used in previous papers by Allen et al. and Megeath et al. to distinguish between protostars, T Tauri stars with disks, and young stars without (inner) disks. The observed IRAC colors are in good agreement with recent improved disk models, and in general accord with models for protostellar envelopes derived from analyzing a larger wavelength region. We also comment on a few Taurus objects of special interest. Our results should be useful for interpreting IRAC results in other, less well studied star-forming regions.

Measuring accretion in young substellar objects: Approaching the planetary mass regime

Abstract: We present observations of Hα emission-line profiles taken at Magellan Observatory for a sample of 39 young low-mass stars and brown dwarfs in the Taurus and Chamaeleon I star-forming regions. We have identified 11 new substellar accretors, more than tripling the number of known brown dwarfs with measurable accretion activity. These include the lowest-mass objects yet seen with accretion, with masses down to ~0.015 M☉. Using models of Hα emission produced in magnetospheric accretion flows, the most widely applicable primary calibrator now available, we determine the first estimates of mass accretion rates for objects at such extremely low masses. For the six objects with masses 0.03 M☉, we find accretion rates of ~5 × 10-12 M☉ yr-1, among the smallest yet measured. These new results continue the trend of decreasing mass accretion rate with decreasing (sub)stellar mass that we have noted previously for samples of more massive objects; the overall correlation is ∝ M2.1, and it now extends over a mass range of over 2 orders of magnitude. Finally, the absence of a discontinuity in the distribution of accretion rates with mass tends to suggest that stars and brown dwarfs share similar formation histories.

The Truncated Disk of CoKu Tau/4

Abstract: We present a model of a dusty disk with an inner hole that accounts for the Spitzer Space Telescope Infrared Spectrograph observations of the low-mass pre-main-sequence star CoKu Tau/4. We have modeled the mid-infrared spectrum (between 8 and 25 μm) as arising from the inner wall of a disk. Our model disk has an evacuated inner zone of radius ~10 AU, with a dusty inner "wall" of half-height ~2 AU that is illuminated at normal incidence by the central star. The radiative equilibrium temperature decreases from the inner disk edge outward through the optically thick disk; this temperature gradient is responsible for the emission of the silicate bands at 10 and 20 μm. The observed spectrum is consistent with being produced by Fe-Mg amorphous glassy olivine and/or pyroxene, with no evidence of a crystalline component. The mid-infrared spectrum of CoKu Tau/4 is reminiscent of that of the much older star TW Hya, where it has been suggested that the significant clearing of its inner disk is due to planet formation. However, no inner disk remains in CoKu Tau/4, consistent with the star being a weak-emission (nonaccreting) T Tauri star. The relative youth of CoKu Tau/4 (~1 Myr) may indicate much more rapid planet formation than typically assumed.

Herbig Ae/Be Stars in nearby OB Associations

Abstract: We have carried out a study of the early-type stars in nearby OB associations spanning an age range of ~3–16 Myr, with the aim of determining the fraction of stars that belong to the Herbig Ae/Be class. We studied the B, A, and F stars in the nearby (≤500 pc) OB associations Upper Scorpius, Perseus OB2, Lacerta OB1, and Orion OB1, with membership determined from Hipparcos data. We also included in our study the early-type stars in the Trumpler 37 cluster, part of the Cep OB2 association. We obtained spectra for 440 Hipparcos stars in these associations, from which we determined accurate spectral types, visual extinctions, effective temperatures, luminosities and masses, using Hipparcos photometry. Using colors corrected for reddening, we find that the Herbig Ae/Be stars and the classical Be (CBe) stars occupy clearly different regions in the JHK diagram. Thus, we use the location on the JHK diagram, as well as the presence of emission lines and of strong 12 μm flux relative to the visual, to identify the Herbig Ae/Be stars in the associations. We find that the Herbig Ae/Be stars constitute a small fraction of the early-type stellar population even in the younger associations. Comparing the data from associations with different ages and assuming that the near-infrared excess in the Herbig Ae/Be stars arises from optically thick dusty inner disks, we determined the evolution of the inner disk frequency with age. We find that the inner disk frequency in the age range 3–10 Myr in intermediate-mass stars is lower than that in the low-mass stars (<1 M⊙); in particular, it is a factor of ~10 lower at ~3 Myr. This indicates that the timescales for disk evolution are much shorter in the intermediate-mass stars, which could be a consequence of more efficient mechanisms of inner disk dispersal (viscous evolution, dust growth, and settling toward the midplane).

Formation of Structure in Molecular Clouds: A Case Study

Abstract: Molecular clouds (MCs) are highly structured and turbulent. Colliding gas streams of atomic hydrogen have been suggested as a possible source of MCs, imprinting the filamentary structure as a consequence of dynamical and thermal instabilities. We present a two-dimensional numerical analysis of MC formation via converging H I flows. Even with modest flow speeds and completely uniform inflows, nonlinear density perturbations arise as possible precursors of MCs. Thus, we suggest that MCs are inevitably formed with substantial structure, e.g., strong density and velocity fluctuations, which provide the initial conditions for subsequent gravitational collapse and star formation in a variety of Galactic and extragalactic environments.

Towards Planetesimals in the Disk around TW Hya: 3.5 centimeter Dust Emission

Abstract: We present Very Large Array observations at 3.5 cm of the nearby young star TW Hya that show the emission is constant in time over weeks, months and years, and spatially resolved with peak brightness temperature ~10 K at ~0.25 (15 AU) resolution. These features are naturally explained if the emission mechanism at this wavelength is thermal emission from dust particles in the disk surrounding the star. To account quantitatively for the observations, we construct a self-consistent accretion disk model that incorporates a population of centimeter size particles that matches the long wavelength spectrum and spatial distribution. A substantial mass fraction of orbiting particles in the TW Hya disk must have agglomerated to centimeter size. These data provide the first clear indication that dust emission from protoplanetary disks may be observed at centimeter wavelengths, and that changes in the spectral slope of the dust emission may be detected, providing constraints on dust evolution and the planet formation process.

The Near-Infrared Size-Luminosity Relations for Herbig Ae/Be Disks

Abstract: We report the results of a sensitive K-band survey of Herbig Ae/Be disk sizes using the 85 m baseline Keck Interferometer. Targets were chosen to span the maximum range of stellar properties to probe the disk size dependenceonluminosityandeffectivetemperature.Formosttargets,themeasurednear-infraredsizes(rangingfrom0.2to 4AU)supportasimple diskmodelpossessingacentralopticallythin(dust-free) cavity,ringedbyhotdustemitting at theexpected sublimation temperatures (Ts � 1000–1500 K).Furthermore, wefindatightcorrelation of disksizewith source luminosity R / L 1 =2 for Ae and late Be systems (valid over more than two decades in luminosity), confirming earlier suggestions based on lower quality data. Interestingly, the inferred dust-free inner cavities of the highest luminosity sources (Herbig B0–B3 stars) are undersized compared to predictions of the ‘‘optically thin cavity’’ model, likely because of optically thick gas within the inner AU. Subject headingg accretion, accretion disks — circumstellar matter — instrumentation: interferometers — radiative transfer — stars: formation — stars: pre–main-sequence

Toward Planetesimals in the Disk around TW Hydrae: 3.5 Centimeter Dust Emission

Abstract: We present Very Large Array observations at λ = 3.5 cm of the nearby young star TW Hya that show the emission is constant in time over weeks, months, and years, and spatially resolved with peak brightness temperature ~10 K at ~025 (15 AU) resolution. These features are naturally explained if the emission mechanism at this wavelength is thermal emission from dust particles in the disk surrounding the star. To account quantitatively for the observations, we construct a self-consistent accretion disk model that incorporates a population of centmeter-size particles that matches the long-wavelength spectrum and spatial distribution. A substantal mass fraction of orbiting particles in the TW Hya disk must have agglomerated to centmeter size. These data provide the first clear indication that dust emission from protoplanetary disks may be observed at centmeter wavelengths, and that changes in the spectral slope of the dust emission may be detected, providing constraints on dust evolution and the planet formation process.

The CIDA variability survey of Orion OB1. I. The low-mass population of Ori OB1a and 1B

Abstract: We present results of a large-scale, multiepoch optical survey of the Orion OB1 association, carried out with the QUEST camera at the Venezuela National Astronomical Observatory. We identify for the first time the widely spread low-mass, young population in the Ori OB1a and OB1b subassociations. Candidate members were picked up by their variability in the V band and position in color-magnitude diagrams. We obtained spectra to confirm membership. In a region spanning ~68 deg2, we found 197 new young stars; of these, 56 are located in the Ori OB1a subassociation and 141 in Ori OB1b. The spatial distribution of the low-mass young stars is spatially coincident with that of the high-mass members but suggests a much sharper edge to the association. Comparison with the spatial extent of molecular gas and extinction maps indicates that the subassociation Ori OB1b is concentrated within a ringlike structure of radius ~2°(~15 pc at 440 pc), centered roughly on the star Ori in the Orion belt. The ring is apparent in 13CO and corresponds to a region with an extinction AV ≥ 1. The stars exhibiting strong Hα emission, an indicator of active accretion, are found along this ring, whereas the center is populated with weak Hα-emitting stars. In contrast, Ori OB1a is located in a region devoid of gas and dust. We identify a grouping of stars within a ~3 deg2 area located in Ori OB1a, roughly clustered around the B2 star 25 Ori. The Herbig Ae/Be star V346 Ori is also associated with this grouping, which could be an older analog of σ Ori. Using several sets of evolutionary tracks, we find an age of 7–10 Myr for Ori OB1a and of ~4–6 Myr for Ori OB1b, consistent with previous estimates from OB stars. Indicators such as the equivalent width of Hα and near-IR excesses show that the number of accreting low-mass stars decreases sharply between Ori OB1b and Ori OB1a. These results indicate that although a substantial fraction of accreting disks remain at ages ~5 Myr, inner disks are essentially dissipated by 10 Myr.

Cepheus OB2: Disk Evolution and Accretion at 3-10 Myr*

Abstract: We presented the results of MMT observations of young stars for our study of protoplanetary disks at ages 1-10 Myr in Tr 37 and NGC 7160. Using low-resolution optical spectra from the Hectospec multifiber spectrograph, we have tripled the number of known low-mass cluster members, identifying 130 new members in Tr 37 and 30 in NGC 7160. We used indicators of youth to identify and classify the low-mass cluster members. With the extended samples, we estimated the disk fraction in the clusters, finding that 40% of the low-mass stars in Tr 37 are actively accreting, whereas only 1 of the 55 low-mass stars in NGC 7160 shows indications of accretion. Optical photometry and theoretical isochrones are used to refine the ages of the clusters, confirming the estimates of 4 Myr for Tr 37 and 10 Myr for NGC 7160.

Discovery of a Planetary-Mass Brown Dwarf with a Circumstellar Disk

Abstract: Using the Hubble Space Telescope, the 4 m Blanco Telescope at the Cerro Tololo Inter-American Observatory, and the Spitzer Space Telescope, we have performed deep imaging from 0.8 to 8 μm of the southern subcluster in the Chamaeleon I star-forming region. In these data, we have discovered an object, Cha 110913-773444, whose colors and magnitudes are indicative of a very low mass brown dwarf with a circumstellar disk. In a near-infrared spectrum of this source obtained with the Gemini Near-Infrared Spectrograph, the presence of strong steam absorption confirms its late-type nature (M9.5) while the shapes of the H- and K-band continua and the strengths of the Na I and K I lines demonstrate that it is a young, pre-main-sequence object rather than a field dwarf. A comparison of the bolometric luminosity of Cha 110913-773444 to the luminosities predicted by the evolutionary models of Chabrier & Baraffe and Burrows and coworkers indicates a mass of 8MJ, placing it fully within the mass range observed for extrasolar planetary companions (M 15MJ). The spectral energy distribution of this object exhibits mid-infrared excess emission at λ > 5 μm, which we have successfully modeled in terms of an irradiated viscous accretion disk with 10-12 M☉ yr-1. Cha 110913-773444 is now the least massive brown dwarf observed to have a circumstellar disk, and indeed is one of the least massive free-floating objects found to date. These results demonstrate that the raw materials for planet formation exist around free-floating planetary-mass bodies.

The Disk Fractions of Brown Dwarfs in IC 348 and Chamaeleon I

Abstract: Using the Infrared Array Camera (IRAC) aboard the Spitzer Space Telescope, we have obtained mid-infrared photometry for 25 and 18 low-mass members of the IC 348 and Chamaeleon I star-forming clusters, respectively (>M6, M 0.08 M☉). We find that 42% ± 13% and 50% ± 17% of the two samples exhibit excess emission indicative of circumstellar disks. In comparison, the disk fractions for stellar members of these clusters are 33% ± 4% and 45% ± 7% (M0-M6, 0.7 M☉ M 0.1 M☉). The similarity in the disk fractions of stars and brown dwarfs is consistent with a common formation mechanism and indicates that the raw materials for planet formation are available around brown dwarfs as often as around stars.

Keck Interferometer Observations of Classical and Weak-line T Tauri Stars

Abstract: We present observations of the T Tauri stars BP Tau, DG Tau, DI Tau, GM Aur, LkCa 15, RW Aur, and V830 Tau, using long baseline infrared interferometry at K band (2.2 μm) from the Keck Interferometer. The target sources have a range of mass accretion rates and excess near-infrared emission. The interferometer is most sensitive to extended emission on characteristic size scales of 1-5 mas. All sources show evidence for resolved K-band emission on these scales, although a few of the sources are marginally consistent with being unresolved. We calculate the infrared excess based on fitting stellar photosphere models to the optical photometry and estimate the physical size of the emission region using simple geometric models for the sources with a significant infrared excess. Assuming that the K-band-resolved emission traces the inner edge of the dust disk, we compare the measured characteristic sizes to predicted dust sublimation radii and find that the models require a range of dust sublimation temperatures and possibly optical depths within the inner rim to match the measured radii.

Accretion, Kinematics, and Rotation in the Orion Nebula Cluster: Initial Results from Hectochelle*

Abstract: We present results from high-resolution spectra of 237 stars in the Orion Nebula cluster (ONC) obtained during two engineering runs with the Hectochelle multifiber echelle spectrograph on the 6.5 m MMT. The ONC is the nearest populous young (age ~ 1 Myr) cluster and is therefore an important object for studies of the evolution of protoplanetary disks. Using the high spectral resolution of Hectochelle, we are able to distinguish stellar accretion and wind emission-line profiles from nebular emission lines and identify accreting members of the cluster from H? profiles with greater accuracy than previously possible. We find 15 new members on the basis of Li ?6707 absorption and H? emission. Line profiles of H? of some objects that are not too contaminated by nebular emission show features characteristic of mass inflow and ejection. We also present rotational velocities as part of an initial investigation into angular momentum evolution of very young stars, confirming a difference between classical T Tauri stars and weak-line T Tauri stars that had been found from period analysis. Finally, we present an initial study of the radial velocity dispersion of the brighter stars in the central cluster. The very small dispersion derived, ?1.8 km s-1, is in good agreement with estimates from proper motions.

The extreme T Tauri star RW Aur: accretion and outflow variability

Abstract: We present an analysis of the classical T Tauri star RW Aur A, based on 77 echelle spectra obtained at Lick Observatory over a decade of observations. RW Aur, which has a higher than average mass accretion rate among T Tauri stars, exhibits permitted (Hα ,H β, Ca II, He I, NaD) and forbidden ((OI)6300 A) emission lines with strong variability. The permitted lines display multiple periodicities over the years, often with variable accretion (redshifted) and outflow (blueshifted) absorption components, implying that both processes are active and changing in this system. The broad components of the different emission lines exhibit correlated behavior, indicating a common origin for all of them. We compute simple magneto- spheric accretion and disk-wind Hα ,H β and NaD line profiles for RW Aur. The observed Balmer emission lines do not have magnetospheric accretion line profiles. Our modeling indicates that the wind contribution to these line profiles is very impor- tant and must be taken into account. Our results indicate that the Hα ,H β and NaD observed line profiles of RW Aur are better reproduced by collimated disk-winds starting from a small region near the disk inner radius. Calculations were performed in a region extending out to 100 R� . Within this volume, extended winds originating over many stellar radii along the disk are not able to reproduce the three lines simultaneously. Strongly open-angled winds also generate profiles that do not look like the observed ones. We also see evidence that the outflow process is highly dynamic - the low- and high-velocity components of the (OI)(6300 A) line vary independently on timescales of days. The apparent disappearance from December 1999 to December 2000 of the (OI)(6300 A) low velocity component, which is thought to come from the disk-wind, shows that the the slow wind can exhibit dramatic variability on timescales of months (placing limits on how extended it can be). There is no comprehensive explanation yet for the behavior of RW Aur, which may in part be due to complications that would be introduced if it is actually a close binary.

Spitzer/IRAC Photometry of the η Chameleontis Association

Abstract: We present IRAC 3.6, 4.5, 5.8, and 8 μm photometry for the 17 A-, K-, and M-type members of the η Chameleontis association. These data show infrared excesses toward six of the 15 K and M stars, indicating the presence of circumstellar disks around 40% of the stars with masses of 0.1-1 M☉. The two A stars show no infrared excesses. The excess emission around one of the stars is comparable to the median excess for classical T Tauri stars in the Taurus association; the remaining five show comparatively weak excess emission. Taking into account published Hα spectroscopy that shows that five of the six stars are accreting, we argue that the disks with weak mid-infrared excesses are disks in which the inner disks have been largely depleted of small grains by grain growth or, in one case, the small grains have settled to the midplane. This suggests that η Cha has a much higher fraction of disks caught in the act of transitioning into optically thin disks than that measured in younger clusters and associations.

Identifying Primordial Substructure in NGC 2264

Abstract: We present new Spitzer Space Telescope observations of the young cluster NGC2264. Observations at 24 micron with the Multiband Imaging Photometer has enabled us to identify the most highly embedded and youngest objects in NGC2264. This letter reports on one particular region of NGC2264 where bright 24 micron sources are spatially configured in curious linear structures with quasi-uniform separations. The majority of these sources (~60% are found to be protostellar in nature with Class I spectral energy distributions. Comparison of their spatial distribution with sub-millimeter data from Wolf-Chase (2003) and millimeter data from Peretto et al. (2005) shows a close correlation between the dust filaments and the linear spatial configurations of the protostars, indicating that star formation is occurring primarily within dense dusty filaments. Finally, the quasi-uniform separations of the protostars are found to be comparable in magnitude to the expected Jeans length suggesting thermal fragmentation of the dense filamentary material.

Disk evolution in the orion ob1 association

Abstract: We analyze multiband photometry of a subsample of low-mass stars in the associations Ori OB1a and 1b discovered during the Centro de Investigaciones de Astronom?a (CIDA) Orion Variability Survey, which have ages of 7?10 and 3?5 Myr, respectively. We obtained UBVRCIC photometry at Mount Hopkins for six classical T Tauri stars (CTTSs) and 26 weak T Tauri stars (WTTSs) in Ori OB1a and for 21 CTTSs and two WTTSs in Ori OB1b. We also obtained L-band photometry for 14 CTTSs at Mount Hopkins and 10 and 18 ?m photometry with OSCIR at Gemini for six CTTSs; of these, all six were detected at 10 ?m, whereas only one was detected at 18 ?m. We estimate mass accretion rates from the excess luminosity at U and find that they are consistent with determinations for a number of other associations, with or without high-mass star formation. The observed decrease of mass accretion rate with age is qualitatively consistent with predictions of viscous evolution of accretion disks, although other factors can also play a role in slowing accretion rates. We compare the excesses over photospheric fluxes in H - K, K - L, and K - N with the younger sample of Taurus and find an overall decrease of disk emission from Taurus to Ori OB1b to Ori OB1a. This decrease implies that significant grain growth and settling toward the midplane has taken place in the inner disks of Ori OB1. We compare the spectral energy distribution of the star detected at both 10 and 18 ?m with disk models for similar stellar and accretion parameters. We find that the low fluxes shortward of 18 ?m of this Ori OB1b star cannot be due to the smaller disk radius expected from viscous evolution in the presence of the far-ultraviolet radiation fields from the OB stars in the association. Instead, we find that the disk of this star is essentially a flat disk, with little if any flaring, indicating a significant degree of dust settling toward the midplane, as expected from dust evolution in protoplanetary disks.

Colors of classical t tauri stars in taurus derived from spitzer infrared spectrograph spectra: indication of dust settling

Abstract: We analyzed Spitzer Infrared Spectrograph data of a representative sample of classical T Tauri stars in the Taurus star-forming region by computing color indices in wave bands that characterize the continuum emission from dust in circumstellar disks. We compared these indices to those derived from a grid of accretion disk models with varying inclination angles, mass accretion rates, and amounts of dust settling. We conclude that most T Tauri stars in our sample have experienced some degree of dust settling and grain growth in their disks, indicating disk evolution.

Spitzer identification of the least massive known brown dwarf with a circumstellar disk

Abstract: Using the Infrared Array Camera (IRAC) aboard the Spitzer Space Telescope, we have obtained mid-infrared photometry of the least massive known brown dwarf in the Chamaeleon I star-forming region. For this young brown dwarf, OTS 44, we have constructed a spectral energy distribution (SED) from 0.8 to 8 μm by combining the measurements at 3.6, 4.5, 5.8, and 8.0 μm from IRAC with ground-based photometry at I, J, H, and K. The resulting SED for OTS 44 exhibits significant excess emission longward of 3 μm relative to the SED expected from the photosphere of the brown dwarf. We have successfully modeled the source of this excess emission in terms of an irradiated viscous accretion disk with 10-10 M☉ yr-1. With a spectral type of M9.5 and a mass of ~15MJup, OTS 44 is now the coolest and least massive brown dwarf observed to have a circumstellar disk. These measurements demonstrate that disks exist around brown dwarfs even down to the deuterium-burning mass limit and the approximate upper mass limit of extrasolar planetary companions.

X-ray Diagnostics of Grain Depletion in Matter Accreting onto T Tauri Stars

Abstract: Recent analysis of high-resolution Chandra X-ray spectra has shown that the Ne/O abundance ratio is remarkably constant in stellar coronae. Based on this result, we point out the utility of the Ne/O ratio as a discriminant for accretion-related X-rays from T Tauri stars and for probing the measure of grain depletion of the accreting material in the inner disk. We apply the Ne/O diagnostic to the classical T Tauri stars BP Tau and TW Hya—the two stars found to date whose X-ray emission appears to originate, at least in part, from accretion activity. We show that TW Hya appears to be accreting material that is significantly depleted in O relative to Ne. In contrast, BP Tau has an Ne/O abundance ratio consistent with that observed for post-T Tauri stars. We interpret this result in terms of the different ages and evolutionary states of the circumstellar disks of these stars. In the young BP Tau disk (age ~0.6 Myr), dust is still present near the disk corotation radius and can be ionized and accreted, rereleasing elements depleted onto grains. In the more evolved TW Hya disk (age ~10 Myr), evidence points to ongoing coagulation of grains into much larger bodies, and possibly planets, that can resist the drag of inward-migrating gas, and the accreting gas is consequently depleted of grain-forming elements.

Wide-Angle Wind-driven Bipolar Outflows: High-Resolution Models with Application to Source I of the Becklin-Neugebauer/Kleinmann-Low OMC-I Region

Abstract: We carry out high-resolution simulations of the inner regions of a wide-angle wind-driven bipolar outflow using an adaptive mesh refinement code. Our code follows H-He gas with molecular, atomic, and ionic components and the associated time-dependent molecular chemistry and ionization dynamics with radiative cooling. Our simulations explore the nature of the outflow when a spherical wind expands into a rotating, collapsing envelope. We compare these with key observational properties of the outflow system of source I in the BN/KL region. Our calculations show that the wind evacuates a bipolar outflow cavity in the infalling envelope. We find the head of the outflow to be unstable and that it rapidly fragments into clumps. We resolve the dynamics of the strong shear layer, which defines the side walls of the cavity. We conjecture that this layer is the likely site of maser emission and examine its morphology and rotational properties. The shell of swept-up ambient gas that delineates the cavity edge retains its angular momentum. This rotation is roughly consistent with that observed in the source I SiO maser spots. The observed proper motions and line-of-sight velocity are approximately reproduced by the model. The cavity shell at the base of the flow assumes an X-shaped morphology that is also consistent with source I. We conclude that the wide opening angle of the outflow is evidence that a wide-angle wind drives the source I outflow and not a collimated jet.

Effects of Dust Growth and Settling in T Tauri Disks

Abstract: We present self-consistent disk models for T Tauri stars which include a parameterized treatment of dust settling and grain growth, building on techniques developed in a series of papers by D'Alessio etal. The models incorporate depleted distributions of dust in upper disk layers along with larger-sized particles near the disk midplane, as expected theoretically and as we suggested earlier is necessary to account for mm-wave emission, SEDs, scattered light images, and silicate emission features simultaneously. By comparing the models with recent mid- and near-IR observations, we find that the dust to gas mass ratio of small grains at the upper layers should be < 10 % of the standard value. The grains that have disappeared from the upper layers increase the dust to gas mass ratio of the disk interior; if those grains grow to maximum sizes of the order of mm during the settling process, then both the millimeter-wave fluxes and spectral slopes can be consistently explained. Depletion and growth of grains can also enhance the ionization of upper layers, enhancing the possibility of the magnetorotational instability for driving disk accretion.

The Accretion Disk of the Lithium-depleted Young Binary St 34

Abstract: We presented the infrared spectrum of the young binary system St 34 obtained with the Infrared Spectrograph (IRS) on the Spitzer Space Telescope. The IRS spectrum clearly shows excess dust emission, consistent with the suggestion of White & Hillenbrand that St 34 is accreting from a circumbinary disk. The disk emission of St 34 is low in comparison with the levels observed in typical T Tauri stars; silicate features at 10 and 20 microns are much weaker than typically seen in T Tauri stars; and excess emission is nearly absent at the shortest wavelengths observed. These features of the infrared spectrum suggest substantial grain growth (to eliminate silicate features) and possible settling of dust to the disk midplane (to reduce the continuum excess emission levels), along with a relatively evacuated inner disk, as expected due to gravitational perturbations by the binary system. Although the position of St 34 in the H-R diagram suggests an age of 8f Myr, assuming that it lies at the distance of the Taurus-Auriga molecular clouds, White & Hillenbrand could not detect any Li I absorption, which would indicate a Li depletion age of roughly 25 Myr or more. We suggest that St 34 is closer than the Taurus clouds by about 30-40 pc and has an age roughly consistent with Li depletion models. Such an advanced age would make St 34 the oldest known low-mass pre-main-sequence object with a dusty accretion disk. The persistence of optically thick dust emission well outside the binary orbit may indicate a failure to make giant planets that could effectively remove dust particles.

Keck Interferometer Observations of FU Orionis Objects

Abstract: We present new K-band long baseline interferometer observations of three young stellar objects of the FU Orionis class, V1057 Cyg, V1515 Cyg and Z CMa-SE, obtained at the Keck Interferometer during its commissioning science period. The interferometer clearly resolves the source of near-infrared emission in all three objects. Using simple geometrical models we derive size scales (0.5-4.5 AU) for this emission. All three objects appear significantly more resolved than expected from simple models of accretion disks tuned to fit the broadband optical and infrared spectro-photometry. We explore variations in the key parameters that are able to lower the predicted visibility amplitudes to the measured levels, and conclude that accretion disks alone do not reproduce the spectral energy distributions and K-band visibilities simultaneously. We conclude that either disk models are inadequate to describe the near-infrared emission, or additional source components are needed. We hypothesize that large scale emission (10s of AU) in the interferometer field of view is responsible for the surprisingly low visibilities. This emission may arise in scattering by large envelopes believed to surround these objects.

Spitzer irs spectra of young stars near the hydrogen-burning mass limit

Abstract: We present Spitzer Infrared Spectrograph measurements for two young stars near the hydrogen-burning mass limit in the Taurus star-forming region. While one of the objects, V410 X-ray 3, displays no mid-infrared excess, the other one, V410 Anon 13, shows a clear excess at wavelengths longward of 10 mm, indicative of a circumstellar disk. Moreover, the disk surrounding V410 Anon 13 is reminiscent of flared accretion disks around classical T Tauri stars; small dust grains in the disk photosphere generate the broad 10 mm silicate emission feature, whose structure suggests the presence of crystalline components. This demonstrates that very low mass objects, like their more massive counterparts, experience dust processing in their disks.

WWW database of models of accretion disks irradiated by the central star

Abstract: We announce the release of a catalog of physical models of irradiated accretion disks around young stars based on the modelling techniques by D’Alessio et al. The WWW catalog includes 3000 disk models for dieren t central stars, disk sizes, inclinations, dust contents and mass accretion rates. For any of them, radial proles of disk physical parameters and synthetic spectral energy distributions can be browsed and downloaded to compare with observations. The catalog can be accessed at http://cfa-www.harvard.edu/youngstars/dalessio/ US), http://www.astrosmo.unam.mx/ dalessio/ (M exico), and at http:// www.laeff.esa.es/models/dalessio/ (Spain).

IRAC Observations of Taurus Pre-Main Sequence Stars

Abstract: We present infrared photometry obtained with the IRAC camera on the Spitzer Space Telescope of a sample of 82 pre-main sequence stars and brown dwarfs in the Taurus star-forming region. We find a clear separation in some IRAC color-color diagrams between objects with and without disks. A few ``transition'' objects are noted, which correspond to systems in which the inner disk has been evacuated of small dust. Separating pure disk systems from objects with remnant protostellar envelopes is more difficult at IRAC wavelengths, especially for objects with infall at low rates and large angular momenta. Our results generally confirm the IRAC color classification scheme used in previous papers by Allen et al. and Megeath et al. to distinguish between protostars, T Tauri stars with disks, and young stars without (inner) disks. The observed IRAC colors are in good agreement with recent improved disk models, and in general accord with models for protostellar envelopes derived from analyzing a larger wavelength region. We also comment on a few Taurus objects of special interest. Our results should be useful for interpreting IRAC results in other, less well-studied star-forming regions.

A Spitzer/IRAC survey of the Orion molecular clouds

Abstract: We present initial results from a survey of the Orion A and B molecular clouds made with the InfraRed Array Camera (IRAC) onboard the Spitzer Space Telescope. This survey encompasses a total of 5.6 square degrees with the sensitivity to detect objects below the hydrogen burning limit at an age of 1 Myr. These observations cover a number of known star forming regions, from the massive star forming clusters in the Orion Nebula and NGC 2024, to small groups of low mass stars in the L1641. We combine the IRAC photometry with photometry from the 2MASS point source catalog and use the resulting seven band data to identify stars with infrared excesses due to dusty disks and envelopes. Using the presence of an infrared excess as an indicator of youth, we show the distribution of young stars and protostars in the two molecular clouds. We find that roughly half of the stars are found in dense clusters surrounding the two regions of recent massive star formation in the Orion clouds, NGC 2024 and the Orion Nebula.